The instant invention relates generally to curable molding compositions and more particularly, to a curable molding composition wherein the vulcanizate comprises primarily previously vulcanized scrap rubber.
Without limiting the present invention, the molding compositions disclosed herein are especially suitable for producing solid rubber tires. As is well known, solid rubber tires are commonly used in conjunction with a wide variety of industrial and consumer products including lawn mowers, tricycles, light duty tractors, hand carts, material handling equipment and so on. With such ultimate uses in mind, it is evident that these solid rubber tires must possess sufficient strength and wear resistant characteristics yet be relatively inexpensive to manufacture.
Prior solid rubber tires were initially manufactured using virgin rubber as the sole or primary source of vulcanizate. For purposes of this application, virgin rubber may be defined as any hydrocarbon rubber polymer which has not previously been subjected to vulcanization. Vulcanization, as is well known, essentially is the treatment of a hydrocarbon rubber polymer in such a manner as to cross-link its long chain molecules with those of a cross-linking agent thereby producing a three-dimensional structure. The result of this reaction is to transform the soft, relatively weak, plastic polymeric material into a relatively strong, elastic material resistant to deterioration normally caused by heat, light and aging process. Vulcanization may be accomplished by a variety of methods, one of the most common being sulfur vulcanization. In this process suitable quantities of sulfur and accelerator are added to the hydrocarbon rubber polymer and the resultant mixture is then subjected to pressure and heat causing the long-chain polymer molecules to cross-link with the sulfur molecules to form three-dimensional molecular structures.
Tires made exclusively from virgin rubber stock possess excellent strength, durability and wear resistant characteristics, however, due to the relatively high cost of such virgin rubber, such tires have proved, from an economic viewpoint, to be commercially prohibitive.
In an effort to produce a tire having the strength and durability of a virgin rubber tire, but at a more commercially acceptable cost, previous attempts have been made to substitute for virgin rubber other less expensive hydrocarbon rubber polymers capable of being vulcanized. One such approach is to employ a form of scrap rubber known as reclaimed rubber as the primary or even the sole source of vulcanizable hydrocarbon rubber polymer. Reclaimed rubber is previously vulcanized rubber which has had fabric and other foreign materials removed and which is then subjected to a devulcanization process. This devulcanization step may be accomplished by a variety of well-known procedures, all intended to depolymerize the rubber molecules without removing any of the combined sulfur. Devulcanization permits subsequent re-vulcanization because it effectively increases the number of unsaturated linkages in the molecular structure of the still cross-linked rubber polymers. Heretofore it had been believed that scrap rubber which did not undergo this devulcanization process was fully saturated and therefore was incapable of further cross-linking. Thus, manufacturers of solid rubber tires are able to re-vulcanize this reclaim by itself or mixed with virgin rubber, to form molded solid rubber tires. Tires made either exclusively or in part from reclaimed rubber stock generally perform satisfactorily and are lower in cost than their counterparts produced exclusively from virgin rubber.
However, reclaimed rubber especially in powdered or ground form is much softer and more flowable than vulcanized rubber which has not undergone the devulcanization step, for example, thus making processing somewhat difficult. This handling difficulty coupled with the actual cost of the additional devulcanization step has been found to increase rather than decrease the cost of manufacturing solid rubber tires, for example.
A second approach taken by various manufactures of solid rubber tires is to employ molding compositions comprising virgin rubber as the primary source of vulcanizable hydrocarbon rubber polymer with the virgin rubber being highly loaded with a relatively inexpensive filler material to reduce the cost. In this regard, conventional scrap rubber was found to serve as an ideal filler material. As used hereinafter, the term scrap rubber means a hydrocarbon rubber polymer which has previously been vulcanized but which has not been subjected to a subsequent devulcanization step, though all fabric and other foreign materials may have been removed. In the known molding process, the scrap rubber filler constituent is ground to a suitable particle size and mixed with virgin rubber, typically in a 1:1 or at most a 2:1 ratio of filler material to virgin rubber. The reason the amount of virgin rubber to the scrap rubber filler ratio is so appreciable, however, is that, heretofore, scrap rubber was believed to be incapable of being revulcanized and subsequently molded into a useful elastomeric product.
Against the foregoing background, it is the primary objective of the present invention to provide a vulcanizable molding composition wherein scrap rubber is utilized primarily as the sole source of vulcanizate.
It is another object of this invention to provide a solid rubber tire fabricated from a molding composition utilizing scrap rubber as the sole source of vulcanizable rubber.
It is an additional object of this invention to provide a solid rubber tire which exhibits physical properties equal or superior to conventional solid rubber tires and which can be manufactured at greatly reduced cost.
Still other objectives and advantages will be made apparent from a study of the following detailed explanation of the invention in connection with the sole accompanying drawing which constitutes a flow diagram diagrammatically indicating the manner in which solid rubber tires according to the present invention may be fabricated.